FIG. 1 shows part of a telecommunications network 2. The network 2 comprises a radio base station 4, which transmits signals to a mobile terminal 6 in downlink communications 8a, and receives signals transmitted by the mobile terminal 6 in uplink communications 8b. The radio base station 4 further communicates with a core network 8.
Those skilled in the art will appreciate that numerous devices and features have been omitted from the description of the network 2 for the purposes of clarity. For example, the network 2 will in general comprise a plurality of radio base stations, with each radio base station transmitting to a plurality of mobile terminals. Further, the core network 8 in general comprises a multitude of different devices, which act to control the operation of the network 2, and to pass data, etc from one part of the network to another.
Both downlink communications 8a and uplink communications 8b are transmitted over a number of different channels, generally including data channels, for transmitting data, and control channels, for transmitting control signals. That is, the downlink communications 8a take place over at least a downlink data channel and a downlink control channel, and the uplink communications 8b take place over at least an uplink data channel and an uplink control channel. For example, UMTS terrestrial radio access (UTRA) according to Release 8 of the 3GPP specifications—also known as evolved UTRA (E-UTRA) or long term evolution (LTE)—defines the physical downlink shared channel (PDSCH) and the physical downlink control channel (PDCCH), and corresponding uplink channels PUSCH and PUCCH.
The resources for transmitting the various channels are divided into so-called “resource blocks”, which each define a portion of frequency and time. That is, the channels are time-division multiplexed (TDM) and frequency-division multiplexed (FDM). In addition to this, to enable an even greater use of resources, a number of users may be multiplexed on a single resource block by means of code-division multiplexing (CDM).
The uplink data channel (e.g. PUSCH) is shared between a number of different users to enable an efficient use of resources. In order to transmit data over a shared channel, a user must request and be granted an allocation of resources on that channel. Such scheduling requests are sent over the uplink control channel, e.g. PUCCH, if resources for scheduling requests have been configured. Scheduling requests are initiated by a buffer status report (BSR) trigger in the mobile terminal. This happens when some higher-layer mechanism determines that data needs to be delivered. The scheduling requests are transmitted within a defined periodic time slot, with the periodicity of scheduling requests being semi-statically configured by radio resource control (RRC) and ranging from 5 to 80 ms. A mobile terminal does not necessarily have scheduling-request resources (there is also an “off” option) in which case it requests uplink grants by a transmission on the Random Access Channel.
When resources have been assigned on the downlink data channel, e.g. PDSCH, data is transmitted by the radio base station 4 to the mobile terminal 6. Many telecommunications networks employ automatic repeat request (ARC)) or hybrid automatic repeat request (HARQ) schemes, where received data is acknowledged by the receiving party. Thus, where the data was received by the mobile terminal 6, acknowledgement messages are sent over the uplink control channel, e.g. PUCCH. If the data is correctly received, a positive acknowledgement (ACK) is sent; if the data is incorrectly received, a negative acknowledgement (NACK) is sent (or alternatively no acknowledgement message is sent).
When employing multiple-input, multiple-output (MIMO) communications between the mobile terminal 6 and the radio base station 4, more than one data stream may be sent to a single mobile terminal. In this case, individual HARQ processes may be used for each data stream by sending different code words for each stream and acknowledging them (i.e. sending an ACK/NACK) individually.
It is also necessary to obtain knowledge about the current state of the interface between the radio base station 4 and the mobile terminal 6. This is important for the purposes of, amongst other things, link adaptation, precoding in multi-input multi-output (MIMO) transmissions, and transmission power control (TPC). The mobile terminal 6 therefore periodically sends channel quality indicators (CQIs) to the radio base station 4 over the uplink control channel, e.g. PUCCH. The periodicity is configured, as with scheduling requests, by higher layers, and ranges from 2 to 256 ms (or alternatively may be completely “off”). The CQIs may include, for example, the signal strength; the delay spread; the path loss; the Doppler spread; the frequency error; the noise strength; the interference strength; and the signal to interference and noise ratio (SINR).
Thus various messages are transmitted over the uplink control channel, including scheduling requests, ACK/NACKs, and CQIs.
A problem arises when the radio base station 4 has to transmit to, and receive transmissions from, many mobile terminals at the same time. As described above, a combination of TDM, FDM and CDM is used to spread the uplink control channel resources according to the various users' requirements. However, a balance must be struck. Too aggressive an allocation of resources (i.e. such that many mobile terminals transmit simultaneously on the same frequency resource) creates a high risk of an unacceptable error probability for ACKs/NACKs, CQIs and scheduling requests. This in turn decreases the system and/or user throughput (for example, because of the necessity of retransmitting signals which were incorrectly received), and degrades the overall system performance. Conversely, too cautious an allocation of resources (i.e. with relatively few mobile terminals transmitting simultaneously on the same frequency resource) creates a risk of poor resource utilization. Greater resources may then need to be allocated to the uplink control channel itself, thereby decreasing the allocation to data channels and reducing data throughput.